On Mon 02-05-11 12:07:59, Surbhi Palande wrote: > On 04/06/2011 02:21 PM, Dave Chinner wrote: > >On Wed, Apr 06, 2011 at 08:18:56AM +0200, Jan Kara wrote: > >>On Wed 06-04-11 15:40:05, Dave Chinner wrote: > >>>On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote: > >>>>On Fri 01-04-11 10:40:50, Dave Chinner wrote: > >>>>>If you don't allow the page to be dirtied in the fist place, then > >>>>>nothing needs to be done to the writeback path because there is > >>>>>nothing dirty for it to write back. > >>>> Sure but that's only the problem he was able to hit. But generally, > >>>>there's a problem with needing s_umount for unfreezing because it isn't > >>>>clear there aren't other code paths which can block with s_umount held > >>>>waiting for fs to get unfrozen. And these code paths would cause the same > >>>>deadlock. That's why I chose to get rid of s_umount during thawing. > >>> > >>>Holding the s_umount lock while checking if frozen and sleeping > >>>is essentially an ABBA lock inversion bug that can bite in many more > >>>places that just thawing the filesystem. Any where this is done should > >>>be fixed, so I don't think just removing the s_umount lock from the thaw > >>>path is sufficient to avoid problems. > >> That's easily said but hard to do - any transaction start in ext3/4 may > >>block on filesystem being frozen (this seems to be similar for XFS as I'm > >>looking into the code) and transaction start traditionally nests inside > >>s_umount (and basically there's no way around that since sync() calls your > >>fs code with s_umount held). > > > >Sure, but the question must be asked - why is ext3/4 even starting a > >transaction on a clean filesystem during sync? A frozen filesystem, > >by definition, is a clean filesytem, and therefore sync calls of any > >kind should not be trying to write to the FS or start transactions. > >XFS does this just fine, so I'd consider such behaviour on a frozen > >filesystem a bug in ext3/4... > > I had a look at the xfs code for seeing how this is done. > xfs_file_aio_write() > xfs_wait_for_freeze() > vfs_check_frozen() > So xfs_file_aio_write() writes to buffers when the FS is not frozen. > > Now, I want to know what stops the following scenario from happening: > -------------------- > xfs_file_aio_write() > xfs_wait_for_freeze() > vfs_check_frozen() > At this point F.S was not frozen, so the next instruction in the > xfs_file_aio_write() will be executed next. > However at this point (i.e after checking if F.S is frozen) the > write process gets pre-empted and say the _freeze_ process gets > control. > > Now the F.S freezes and the write process gets the control back. And > so we end up writing to the page cache when the F.S is frozen. > -------------------- > > Can anyone please enlighten me on how & why this premption is _not_ > possible? XFS works similarly as ext4 in this regard I believe. They have the log frozen in xfs_freeze() so if the race you describe above happens, either the writing process gets caught waiting for log to unfreeze or it manages to start a transaction and then freezing process waits for transaction to finish before it can proceed with freezing. I'm not sure why is there the check in xfs_file_aio_write()... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
